1. Field of the Invention
The invention refers to a movable trestle of approximately rectangular horizontal projection and serving the purpose of supporting the roof in underground cavities, in particular serving the purpose of securing the excavation edge when recovering pillars in coal mining, comprising a cap supported against a base frame by hydraulically extensible props extending approximately in vertical direction to stratification, noting that shielding means for preventing ingress of excavated material into the interior of the trestle are provided at least at the rear side of the trestle and at both sides of the trestle adjoining its rear side and noting that the lower portion of the trestle is covered by fixedly arranged shielding means and that movable shielding means are provided within the area of the extensible portion of the trestle.
2. Description of the Prior Art
In connection with mine consolidating means being, as a rule, used in immediately adjacent relation, there are known shielding means preventing excavated material from entering the interior cavity of the trestle. There has already been proposed to arrange shields at the rear side of the trestle and at the side surfaces guards having the shape of chain curtains. Such an arrangement does, however, not provide a reliable protection against the ingress of excavated material and this in particular if the excavated material is pushed in direction to the trestle. From DE-OS 1 919 660 it has already become known to provide U-shaped chests embracing the trestle at its rear side and at its both sides adjoining the rear side and being guided for telescoping movement one within the other. In this case, the just overlying chest has a smaller horizontal projection than the just underlying chest and can be shifted into the latter. Such an arrangement is expensive and has a tendency to jamming on account of the telescopic arrangement, and additionally it is undesired in case of some types of mine consolidation that the trestle tapers in upward direction when being expanded.